Carbonaceous luminescent materials have been attracting attention recently as luminescent materials. One type of carbonaceous luminescent material is graphene quantum dots. Graphene quantum dots are expected to be superior to semiconductor quantum dots in terms of, for example, price, safety and chemical stability.
Existing methods for producing carbonaceous luminescent materials are broadly divided into two types: top-down methods and bottom-up methods (Non-Patent Document 1). The top-down approach refers to methods of preparation that use physical and chemical techniques to break down to the nano level bulk substances having a graphite structure, such as carbon fibers, coke or oxidized graphene. Although top-down methods are capable of mass production, narrow-band emission is difficult and so carbonaceous luminescent materials endowed with industrially useful emission characteristics are not manufactured in this way.
The bottom-up approach uses a low-molecular-weight compound as the carbon source and carries out chemical synthesis by firing or a hydrothermal reaction. By applying a suitable dope to the product, a carbonaceous luminescent material that exhibits a very high quantum yield of about 70% or more is produced via a dry process which involves firing cystine as the starting material. However, the yield is very low (Non-Patent Document 2).
In addition, Non-Patent Document 3 describes a bottom-up approach for producing carbonaceous luminescent material by a wet process. However, even when this method was repeatedly carried out under the same reaction conditions, it was not possible to obtain a carbonaceous luminescent material. Hence, the reproducibility of this method is questionable.